The present invention relates to a switching power supply, and more specifically, to a switching power supply that can prevent an output voltage Vout from undershooting and fluctuating when the operation of the switching power supply is stopped.
Switching power supplies are widely used as power supplies for electrical and electronic equipment such as computers.
FIG. 7 is a circuit diagram showing a conventional switching power supply.
As shown in FIG. 7, the conventional switching power supply is composed of a transformer T1, a switching circuit located on the primary side of the transformer T1, and a rectifier of the self-drive type and a smoothing circuit located on the secondary side of the transformer T1. The switching power supply lowers a DC (direct current) input voltage Vin supplied to the switching circuit located on the primary side to generate a DC output voltage Vout and supplies it to a load. In FIG. 7, the load is represented by a resistance component RLoad, capacitance component CLoad, and reactance component LLoad.
A control circuit 10 controls main switches Q1 and Q2 included in the switching circuit of the primary side based on the output voltage Vout. Specifically, the control circuit 10 lowers the duty factor of the main switches Q1 and Q2 when the output voltage Vout increases relative to the desired voltage so as to decrease the electric power supplied to the load and raises the duty factor of the main switches Q1 and Q2 when the output voltage Vout decreases relative to the desired voltage so as to increase the electric power supplied to the load. Thus, the output voltage Vout supplied to the load can be always stabilized at the desired voltage. Because the control circuit 10 belongs to the primary side, the control circuit 10 cannot receive the output voltage Vout directly. The control circuit 10 is therefore supplied via an isolation circuit 20 with a voltage Voutxe2x80x2 associated with the output voltage Vout.
Operating voltage Vcc for the control circuit 10 is generated by an operating voltage generation circuit consisting of a transistor Tr1, resistor R1, and zener diode Z1. A capacitor C3 is connected between power terminals of the control circuit 10 for stabilizing the operating voltage Vcc. The operating voltage generation circuit is activated when an operation switch S1 is in the ON state and inactivated when the operation switch S1 is in the OFF state. The operation switch S1 can be controlled from the outside. When the operation of the switching power supply shown in FIG. 7 is to be started, the operation switch S1 is turned ON; when the operation of the switching power supply is to be terminated, the operation switch S1 is turned OFF.
Rectifying switches Q3 and Q4 included in the rectifier of the secondary side are self-driven by the secondary voltage of the transformer T1. Further, resistors R2 and R3 are inserted between the gate electrodes and the source electrodes of the rectifying switches Q3 and Q4, respectively, so as to prevent the gate electrodes of the rectifying switches Q3 and Q4 from being in a floating state.
Next, the operation of the conventional switching power supply shown in FIG. 7 will be explained.
FIG. 8 is a timing chart showing the operation of the conventional switching power supply shown in FIG. 7.
As shown in FIG. 8, when the operation switch S1 is in the ON state, the gate-source voltages VGS(Q1) and VGS(Q2) of the main switches Q1 and Q2 are alternately activated to a high level at a predetermined switching frequency under the control of the control circuit 10. As a result, the polarity of the primary voltage VLP of the transformer T1 is alternately inversed, so that primary side capacitors C1 and C2 are alternately charged and discharged.
Synchronously with the operation of the primary side, the polarity of the secondary voltage appearing at secondary coils Ls1 and Ls2 of the transformer T1 is alternately inversed, so that the rectifying switches Q3 and Q4 are alternately brought into ON state in turn at the predetermined switching frequency. More specifically, while the main switch Q1 is in the ON state owing to the gate-source voltage VGS(Q1) being at a high level, the gate-source voltage VGS(Q3) of the rectifying switch Q3 is raised to a voltage greater than the threshold voltage thereof by the voltage (secondary voltage) appearing at secondary coil Ls2, whereby the rectifying switch Q3 turns ON. On the contrary, while the main switch Q2 is in the ON state owing to the gate-source voltage VGS(Q2) being at a high level, the gate-source voltage VGS(Q4) of the rectifying switch Q4 is raised to a voltage greater than the threshold voltage thereof by the voltage (secondary voltage) appearing at secondary coil Ls1, whereby the rectifying switch Q4 turns ON.
As a result, the secondary voltage of alternately inversed polarity is rectified. The rectified voltage is smoothed by the smoothing circuit, which consists of an output reactor Lout and output capacitor Cout so that the stabilized output voltage Vout is generated.
On the other hand, when the operation switch S1 is turned OFF at a certain time, the operation of the control circuit 10 is stopped because the transistor Tr1 turns OFF, so that both the main switches Q1 and Q2 are put in the OFF state. That is, the switching operation is stopped.
However, because the operation of the switching circuit of the primary side is stopped when the operation switch S1 is turned OFF, one or the other of the rectifying switches Q3 and Q4 is kept in the ON state and a reverse current begins to flow from the output capacitor Cout and the capacitance component CLoad of the load to the output reactor Lout.
FIG. 8 shows the case where the rectifying switch Q3 is kept in the ON state at first in response to the operation switch S1 being turned OFF. In this case, because the switching circuit of the primary side is stopped, the discharge path for the electric charge of the gate electrode of the rectifying switch Q3 is substantially only the resistor R2. Therefore, the gate-source voltage VGS(Q3) of the rectifying switch Q3 falls gradually owing to the current flow through the resistor R2. During this period, the reverse current flowing to the output reactor Lout continues.
On the other hand, when the rectifying switch Q3 turns OFF because the gate-source voltage VGS(Q3) of the rectifying switch Q3 falls below the threshold voltage thereof owing to the decrease of the output voltage Vout and the secondary voltage by discharge of the output capacitor Cout and the capacitance component CLoad of the load and discharge of the electric charge from the gate electrode of the rectifying switch Q3 via resistor R2, a flyback voltage rises at the transformer T1. The flyback voltage boosts an internal voltage Vp in the switching circuit via the transformer T1 and boosts the gate-source voltage VGS(Q4) of the rectifying switch Q4. Therefore, the rectifying switch Q4 stays ON.
As shown in FIG. 8, because the direction of the current flowing to the output reactor Lout via the rectifying switch Q4 becomes forward temporarily, the output capacitor Cout and the capacitance component CLoad of the load are charged during this period, so that the output voltage Vout is increased.
Then, when the direction of the current flowing to the output reactor Lout becomes reverse, the gate-source voltage VGS(Q4) of the rectifying switch Q4 falls gradually owing to the decrease of the output voltage Vout and the secondary voltage by discharge of the output capacitor Cout and the capacitance component CLoad of the load and discharge of the electric charge from the gate electrode of rectifying switch Q4 via resistor R3. Then, when the rectifying switch Q4 turns OFF because the gate-source voltage VGS(Q4) of the rectifying switch Q4 falls below the threshold voltage thereof, the flyback voltage rises again at the transformer T1, which boosts the internal voltage Vp in the switching circuit via the transformer T1 and boosts the gate-source voltage VGS(Q3) of the rectifying switch Q3. Therefore, the rectifying switch Q3 stays ON.
Such operations are periodically repeated until the output capacitor Cout and the capacitance component CLoad of the load are consumed by the secondary side circuit and the resistance component RLoad of the load. Therefore, the output voltage Vout gradually decreases while fluctuating over very long period compared with the switching period and, in addition, the internal voltage Vp in the switching circuit is gradually increased.
As described above, in the conventional switching power supply, because the output voltage Vout does not decrease linearly but falls gradually while fluctuating over very long period compared with the switching period even if an instruction to stop the operation of the switching power supply is issued (the switch S1 is turned OFF), some malfunction may arise in the load. For example, the load may be designed to discriminate when the operation of the switching power supply has stopped and perform a certain operation when the output voltage Vout falls below a predetermined voltage. But if the output voltage Vout gradually decreases while fluctuating, discriminating whether the switching power supply as stopped becomes difficult.
Further, in the conventional switching power supply, because the internal voltage Vp in the switching circuit gradually increases during termination of operation, electric components used on the primary side may be damaged. In order to prevent this, components having a high withstand voltage must be used. This increases the cost of the switching power supply.
Furthermore, in the conventional switching power supply, because large current flows through the output reactor Lout, the secondary coils Ls1 and Ls2 of the transformer T1 and the rectifying switches Q3 and Q4 during termination of operation, the reliability of the switching power supply may be degraded because the output reactor Lout, the secondary coils Ls1 and Ls2 of the transformer T1 and the rectifying switches Q3 and Q4 release a large amount of heat.
These problems become more pronounced as the resistance component RLoad of the load becomes larger. Therefore, in the case where the instruction to terminate operation is issued in a light-load condition, the problems are serious. Further, because the problems become more pronounced as the capacitance component CLoad of the load becomes large, the problems are also serious when the electric power is supplied to a load having a large capacitance component CLoad.
On the other hand, although the problems are not so serious when the resistance component RLoad of the load is considerably small (i.e., the load is heavy), in this case, some malfunction may arise in the load during the termination of operation owing to undershoot of the output voltage Vout. For example, when the output voltage Vout becomes negative, parasitic diodes and so forth in an integrated circuit (IC) employed in the load may turn ON. As this causes huge current to flow through the IC, the IC may malfunction or be damaged.
FIG. 9 is a timing chart showing the undershoot of the output voltage Vout during the termination of operation.
As shown in FIG. 9, when the operation of the switching circuit of the primary side is stopped by turning OFF the operation switch S1, the current IRLoad flowing through the resistance component RLoad is changed from the output current ILout of the output reactor Lout to the discharge current ICout of the output capacitor Cout and the voltage VLLoad rises at the reactance component LLoad of the load, so that current continues to flow. As a result, the output voltage Vout becomes negative, i.e., undershoot arises. Then, if the undershoot voltage reaches the forward voltage Vf of the body diodes of the rectifying switches Q3 and Q4, these body diodes turn ON. As a result, current begins to flow through the LCR serial circuit consisting of the rectifying switch Q3 (body diode), the secondary coil Ls1 of the transformer T1, the output reactor Lout, and the output capacitor Cout and another LCR serial circuit consisting of the rectifying switch Q4 (body diode), the secondary coil Ls2 of the transformer T1, the output reactor Lout, and the output capacitor Cout. Therefore, the peak value of the undershoot voltage is clamped to about xe2x88x92Vf.
Here, when the relationship between the resistance component RLoad, the reactance component LLoad, and the output capacitor Cout satisfies the formula (1), these LCR serial circuits oscillate. Undershoot arises as a result.                                           RLoad            ⁢                          xe2x80x83                                2                 less than                   4          ·                      LLoad            Cout                                              (        1        )            
As can be seen from the formula (1), undershoot tends to arise when the resistance component Rload is small (when the load is heavy). In order to prevent the switching power supply from undershooting, an additional capacitor Cex of sufficient capacitance needs to be connected in parallel with the output capacitor Cout because the resistance component RLoad and the reactance component LLoad belong to the load. This leads to an undesirable increase in number of components. The capacitance required by the additional capacitor Cex for preventing undershoot can be represented by formula (2):                               C          EX                 greater than                               4            ·                          LLoad                                                RLoad                  ⁢                                      xe2x80x83                                                  2                                              -          Cout                                    (        2        )            
Because this problem is pronounced when the resistance component Rload is small, it becomes serious when the switching power supply is used to drive a load requiring a low voltage and a large current, such as a server computer.
As explained above, the conventional switching power supply has two main problems: one is that the output voltage Vout falls gradually while fluctuating over a very long period when an instruction for stopping the operation of the switching power supply is issued; and the other is that undershoot arises in the output voltage Vout when the instruction for stopping the operation of the switching power supply is issued. The former problem becomes pronounced when the resistance component Rload is large, while the latter problem becomes pronounced when the resistance component Rload is small. The latter problem arises whether or not the rectifier is a self-drive type.
It is therefore an object of the present invention to provide a switching power supply that prevents the output voltage Vout from undershooting when an instruction for stopping the operation of the switching power supply is issued.
Another object of the present invention is to provide a switching power supply that prevents the output voltage Vout from fluctuating when an instruction for stopping the operation of the switching power supply is issued.
A further object of the present invention is to provide a switching power supply that prevents the internal voltage Vp of the switching circuit from gradually increasing when an instruction for stopping the operation of the switching power supply is issued.
A still further object of the present invention is to provide a switching power supply that prevents a large amount of current from flowing through the output reactor Lout, the secondary coils Ls1 and Ls2 of the transformer T1 and the rectifying switches Q3 and Q4 when an instruction for stopping the operation of the switching power supply is issued.
Also according to these aspects of the present invention, the switching power supply can lower its output voltage substantially linearly without fluctuating or undershooting. Malfunction of the load can therefore be effectively avoided. Particularly, in the case where the rectifier is of a self-drive type, the internal voltage in the second converter is prevented from gradually increasing when an instruction for stopping the operation of the switching power supply is issued. The electric components used on the primary side are therefore effectively protected from damage. Further, because it is not necessary to use components having high withstand voltage, the cost of the switching power supply can be lowered. Furthermore, because a large current does not flow thorough the output reactor, the secondary coil of the transformer and the rectifier when the instruction is issued, the reliability of the switching power supply can be enhanced.
The above and other objects and features of the present invention will become apparent from the following description made with reference to the accompanying drawings.